Stabilized cellulose lower alkanoates



Patented Sept. 15, 1953 STABILIZED CELLULOSE LOWER ALKANOATES Gordon Hiatt and R ester, N. Y., assignors Jersey I bert F. Williams, Rochto Eastman Kodak Company, Rochester, N. Y., a corporation of New No Drawing. Application June 4, 1949, Serial No. 97,325

8 Claims. a 1

This invention relates to stabilized cellulose esters having low ash and sulfur contents which have been stabilized by the addition thereto of an alkali metal acid salt of oxalic, tartaric or malic acid.

Cellulose esters as prepared in the conventional manner from woodpulp or cotton linters tend to discolor at high temperatures and undergo chain cleavage. This is a decided disadvantage in the use of such esters in processes involving elevated temperatures, such as in molding, fabrics, etc., and, consequently, methods have been developed for stabilizing those esters against the effects of elevated temperatures. One method for retarding discoloration has been the addition of weak acids, but the gain in color stability obtained by this addition has usually been offset by a loss in the chain length stability of the cellulose ester. In some casessalts have been added to the cellulose esters but in those cases the color stability and clarity have often been sacrificed even though chain length stability has been achieved.

One object of our invention is stabilized cellulose esters which retain color and chain length stability at elevated temperatures and which, also, are of excellent clarity. Another object of our invention is to provide a method of stabilizing cellulose esters utilizing salts of oxalic, tartaric, and malic acids. Other objects of our invention will appear herein.

Ordinarily the haze in cellulose esters is the result of the presence'of minerals in the material. In order to stabilize such esters by the addition of more mineral material, the combination of clarity, good color stability and good chain length stability is difficult to achieve. We have found, however, that all three properties may be obtained in the same cellulose ester if the ester is prepared under such conditions that the hydrolysis thereof is performed in the presence of only a small proportion of sulfateion and the ester is washed with water of low mineral content, followed by treatment thereof with acid salt as described above in a proportion related to the sulfur content which is present.

In its broadest aspects our invention involves the esterification of cellulose with a lower fatty acid anhydride, such as acetic anhydride or butyric anhydride in the presence of sulfuric acid catalyst, neutralization at the end of the esterification of .25 to .9 of the esterification catalyst with a neutralizing agent which forms a salt which is insoluble in the reaction mixture and adding water or aqueous acid to cause hydrolysis of the cellulose ester at a temperature 2, of 110-180 F. and at a rate that the water is stirred into the mass as it is added. If the stir-, ring is vigorous, the rate of addition may be fairly rapid. After the ester has been hydrolyzed for a time, in the case of cellulose acetate for a time sufficient to give acetone solubility, the ester is filtered to remove amajor portion of the sulfate salt formed by catalyst neutralization with MgO, Mg(C2H3O2)2 or the like, andthen precipitated from solution with aqueous acetic acid, such as of 5-20% concentration and washed at least once with water having a mineral content less than 20 p. p. m. Either in the last wash or after the last wash and prior to drying or to the dried ester duringcompounding with the plasticizer, a water-soluble acid salt of oxalic, malic, or tartaric acid issupplied to the ester in the proportion of l-3 molecular equivalents of the salt per equivalent of sulfate ion based ,on the sulfur content of the ester.

Our invention applies to cellulose esters of fatty acids of 2-4 carbon atoms, either simpleor mixed. Some of the esters which may be prepared in accordance with our invention are cellulose acetate, cellulose acetate butyrate, cellulose acetatepropionate, cellulose butyrate, and cellulose propionate. In the case of cellulose acetates the esterifying liquid is made up primarily of acetic anhydride, acetic acid, and sulfuric acid catalyst. In the case of celluose acetate butyrate the esterifying liquid may be made up either of butyric anhydride, acetic acid and sulfuric acid catalyst or of acetic anhydride, butyric acid, and catalyst depending on. whether a high or low butyryl ester is desired. The pro portion of catalyst employed in the esterification may be from 1.5 to 10% of sulfuric acid based on the dry weight of thecellulose. Ordinarily; to obtain esters of good viscosity the esterification temperature is not allowed to rise above F. although the esterification temperatures employed depend somewhat upon the amount of sulfuric acid catalyst used, the more catalyst present in the esterification mass, the more important it becomes that the temperature of the reaction mass be carefully controlled. In order to terminate the esterification, water, usually in the form of aqueous acid, is added to the reaction mass in a sufficient proportion to convert the acetic anhydride remaining to acetic acid. In this addition, or in the water in the form of aqueous acid which is added to initiate the hy drolysis, or as a separate addition at this time, a neutralizing agent is applied to the esterwhich will form a salt of a considerableproport'o of 3 the sulfuric acid, which salt is insoluble in the reaction mixture. Magnesia or magnesium compounds of weak acids have been found to be especially useful for this purpose as the magnesium sulfate formed is insoluble inthe re action mass. Instead, .a calcium'neutralizing agent can be employed in this connection, if desired. By neutralizing the greater portion, but not all, of the catalyst which is present, the hydrolysis may be carried out but the combined sulfuric acid in the final content in the final product-of; .001.-to .015%. In carrying out the hydrolysis ofthescellulosecester the water in the form of aqueous acid which: is

added to the mass should be added at such a rate that it is uniformly worked into .the hydrolysis mixture. Also, the addition and the hydrolysis should be at a temperature within the range of 110-l80 F. After the ester has been hydrolyzed to the desired extent which is ordinarily to impart.05'-5 hydroxylscpen Cs cellulose .units, the ester is-precipitated:;by.the addition ohaqueous acid to 1 recover the. ester in..a powdered form. The ester isthen given a. thorough washing in water, preferably.severalwasheswith water .having a low mineralacontentsuch asless than:.20

p..m. mineral. content. Such water maybe either distilled water or. water which hasbeen prepared using Permutiteor other. means which removes ions .fromthe water available for washing purposes. LFIhe.. use.oiE water having a mineral content of about .10. :p.:.p. m. has-beenv found to be-quite. satisfactory and with three or four washings with thistypeof .water theash'content of celluloseestershas.geen reduced to 1315-04770.

The iester should be awashedsuific'iently that the ash :content thereof. does .not exceed 1.05%

In-the last wash,..or. asa separate addition after the last-wash andsbefore .drying or as, an

addition to the compounding mixer,;.an:acid salt 5 of oxalic, malic, or tartaric .acid-isadded in the form of iitsaqueous-solution. This.;salt..-may .be added in-solution distilled water. orany. other water vof a low. mineral. content. Salts which are suitable-in this connection. may .be either potassium. acid oxalate, potassium: acid. .maleate, or-potassiumacid tartrate, .or.the.;solutions.:.may be made :by -mixing,.rori instance; oxalic acidand potassium: or sodiumioxalate which forms the acid salt. .-Exact.- equivalency not. necessary,.;but it is .desirableata high.stabilizing-salt.level that the equivalency be close,:..not-..varyingcmore.than 5%. Where. only; asmalliamonnt .of:stabilizing salt is necessary,c-one 'ireagent .may exceed. .the

other. by as. much .as =e15 %awithoutzseriously interferring with theclarity orthe. chain length stability ofithe. resulting product. Asa general rule,.it*may be stated-that theratio of the potassium oxalate to.,oxalicacid should. be within the rangeiof' 2.25%.75; 1375-2 .25. Themakeup of the. acid salt-may besgovernedto some extent by the character of the ester being. treated. If the ester. has alkaline .sal-ts contributedsbythe wash water, it may begdesirable .to tend slightly to' the acid; side. On the. other hand, if the ester has a .low salt. contentand contains a little unneutralized sulfuric acid, a slight excess of the neutral salt may be. useful. -For the..usual-,.ester batch; straightgacid .salt ;-may be, used... but stabilization may also be effected. when this salt is toward the: acid-or toward, the alkaline side by as-much as As an illustration ofv the effectiveness ofjour invention a hydrolyzed cellulose: acetate. butyrate ester ;:of-.. a. sulfur, andaash content within the 1M) ester is held to a sulfur ranges specified herein was divided into five parts and those parts were treated with the acid and the neutral salt in varying proportions. The following table shows those proportions ranging 5 from 0 acid to. 0 salt. The. samples. were then heated for one hour at.,20-5 C,., .and tested for color, and for two hours at 205 C. and tested for intrinsic viscosity. The proportions and the intrinsic viscosities are as follows:

Stabilizer ggi Color,lHr.

, Control Arnhem... .92

A r; .-B {EZSZS 18%? --{fii8ijij 1832 W --{ii iii: 135?? W 20 E -{Z88ijji 18% F 1 These. results show. that .onlythestabilizer mp gives .a product .satisfactorybotn as to color .and intrinsic viscosity (cellulose chain length).

.It is necessary .thatthe acid oxalate (orsother equivalent). saltremain in the cellulose ester in order for it to beeif ective. v.For instance, a ,300 g. sample or" .cellulosdacetate .butyratebfjlow ash .and sulfur. content,..in powdered form". was heated. at' 1.-85 C. fortwo ho-urs,in.3000. cc .of,'.d istilled water containing. L .g. of potassium acid oxalate. This, amountbfthe .acid .saltis, equivalentto 1% or .oxalicacid basedon the cellulose ester. The slurry. wasfallowed to. coolLand was then drained. -Thernasswasdivided. into Qqual portions, .onelhalf ..(A) b$ing ..th oroughly' washed in distilled water. and .the.,other half (B) ..bein washed in tap .water. .Both samples were. compounded with..5 of .addedhutyi .sebacate .to assist their plasticity. ,nA sarnple, of ,the, saline ester (C) ,was treatedbythe stabilizing method described herein 1. and was compounded as ..de-- scribed. The-three ,p.lastic,..cornpositions .were heated intesttubes iorpneandfor two hoursat 205l C...and.were tested.fori haze,..color and yiscosity. .Theresults swereasfollowsz 4' (Green a EX. .Gonc. "Color. ,IndeX 107's ,Madeupto oneliter with qimethyr r t aiate Theidark brown solutionthusi obtained .is futth r .di1uted:xwith dimethyl:.phthalata-soythat 1 a; 1.120 ,solution'xis regarded as-havingiacolocof 5 00. ..50

solution as; haying a color pf -l8,0,r.and'.- a 1;

solution: as having acolor; of: 150.

vvflhe.sleaze: is det rminedby:ccmpa so awithsa malic and tartaric in the amount of 1-3 molecular equivalents of the salt per equivalent of sulfate ion in the ester whereby the cellulose ester isstabilized against the effects of elevated temperatures.

:2. A cellulose acetate butyrate containing .05-.5 hydroxyls per Ce unit of cellulose, which ester has a sulfur content of .001-015% and an ash content of not more than .05% in which has been incorporated an alkali metal acid salt of an acid selected from a. group consisting of oxalic, malic and tartaric in .the amount of 1-3 molecular equivalents of the salt per equivalent of sulfate ion in the ester whereby the ester is stabilized against the-effects of elevated temperatures.

a 3.- A cellulose acetate containing .05-.5 hydroxyls per Ce unit ofxcellulose, which ester has a sulfur content of .001-.015% and an ash content of not more than .05% in which has been incorporated an alkali metal acid salt of an acid selected from a group consisting of oxalic, malic and tartaric in the amount of 1-3 molecular equivalents of the salt per equivalent of sulfate ion in the ester whereby the ester is stabilized against the effects of elevated temperatures.

4. A lower fatty acid ester of cellulose containing .05.5 hydroxyls per Cs unit of cellulose, which ester has a sulfur content of .001 %-.015% and an ash content of not more than .05% in which has been incorporated an alkali metal acid oxalate in the amount of 1-3 molecular equivalents of the salt per equivalent of the sulfate ion in the ester whereby the ester is stabilized against the effects of elevated temperatures.

5. A lower fatty acid ester of cellulose containing .05-.5 hydroxyls per Cc unit of cellulose, which ester has a sulfur content of .001%.015% and an ash content of not more than .05% in which has been incorporated a potassium acid oxalate in the amount of 1-3 molecular equivalents of the salt per equivalent of the sulfate ion in the ester whereby the ester is stabilized against the effects of elevated temperatures.

6. A lower fatty'acid ester of cellulose containing .05-.5 hydroxyls per Cs unit of cellulose, which ester has a sulfur content of .001%-.015% and an ash content of not more than .05% in which has been incorporated an alkali metal acid tartrate in the amount of 1-3 molecular equivalents of the salt per equivalent of the sulfate ion in the ester whereby the ester is stabilized against the effects of elevated temperatures.

7. A lower fatty acid ester of cellulose containing .05-.5 hydroxyls per 06 unit of cellulose, which ester has a sulfur content of .001 %-.015% and an ash content of not more than .05% in which has been incorporated an alkali metal acid malate in the amount of 1-3 molceular equivalents of the salt per equivalent of the sulfate ion in the ester whereby the ester is stabilized against the effects ofelevated-temperatures.

8. A thermoplastic composition which forms plastic products of good clarity and color stability at elevated temperatures, essentially consisting of a lower fatty acid ester of cellulose containing .05-.5 hydroxyls per Cs unit cellulose unit, a sulfur content of .001-.015% and an ash content of not more than .05% in which ester has been incorporated an alkali metal acid salt of an acid selected from oxalic, malic and tartaric in'theamount of 1-3 molecular equivalents of the salt per equivalent of sulfate ion in the ester and a plasticizer for the cellulose ester. v

. GORDON D. HIATT.

ROBERT F. WILLIAMS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,607,474 Mork Nov. 16, 1926 1,921,949 Sheppard Aug. 8, 1933 2,091,972 Dreyfus Sept. '7, 1937 2,154,822 Quisling Apr. 18, 1939 2,265,218 Stone Dec. 9, 1941 2,361,509 Rowley Oct. 31, 1944 

1. A LOWER FATTY ACID ESTER OF CELLULOSE CONTAINING .05-5 HYDROXYLS PER C6 UNIT OF CELLULOSE, WHICH ESTER HAS SULFUR CONTENT OF .001-.015% IN WHICH HAS ASH CONTENT OF NOT MORE THAN .05% IN WHICH HAS BEEN INCORPORATED AN ALKALI METAL ACID SALT ON AN ACID SELECTED FROM A GROUP CONSISTING OF OXALIC. MALIC AND TARTARIC IN THE AMOUNT OF 1-3 MOLECULAR EQUIVALENTS OF THE SALT PER EQUIVALENT OF SULFATE ION IN THE ESTER WHEREBY THE CELLULOSE ESTER IS STABILIZED AGAINST THE EFFECTS OF ELEVATED TEMPERATURES. 